On the actual behavior, there is no difference. They all return None and that's it. However, there is a time and place for all of these.
The following instructions are basically how the different methods should be used (or at least how I was taught they should be used), but they are not absolute rules so you can mix them up if you feel necessary to.
Using return None
This tells that the function is indeed meant to return a value for later use, and in this case it returns None. This value None can then be used elsewhere. return None is never used if there are no other possible return values from the function.
In the following example, we return person's mother if the person given is a human. If it's not a human, we return None since the person doesn't have a mother (let's suppose it's not an animal or something).
def get_mother(person):
if is_human(person):
return person.mother
else:
return None
Using return
This is used for the same reason as break in loops. The return value doesn't matter and you only want to exit the whole function. It's extremely useful in some places, even though you don't need it that often.
We've got 15 prisoners and we know one of them has a knife. We loop through each prisoner one by one to check if they have a knife. If we hit the person with a knife, we can just exit the function because we know there's only one knife and no reason the check rest of the prisoners. If we don't find the prisoner with a knife, we raise an alert. This could be done in many different ways and using return is probably not even the best way, but it's just an example to show how to use return for exiting a function.
def find_prisoner_with_knife(prisoners):
for prisoner in prisoners:
if "knife" in prisoner.items:
prisoner.move_to_inquisition()
return # no need to check rest of the prisoners nor raise an alert
raise_alert()
Note: You should never do var = find_prisoner_with_knife(), since the return value is not meant to be caught.
Using no return at all
This will also return None, but that value is not meant to be used or caught. It simply means that the function ended successfully. It's basically the same as return in void functions in languages such as C++ or Java.
In the following example, we set person's mother's name and then the function exits after completing successfully.
def set_mother(person, mother):
if is_human(person):
person.mother = mother
Note: You should never do var = set_mother(my_person, my_mother), since the return value is not meant to be caught.
On the actual behavior, there is no difference. They all return None and that's it. However, there is a time and place for all of these.
The following instructions are basically how the different methods should be used (or at least how I was taught they should be used), but they are not absolute rules so you can mix them up if you feel necessary to.
Using return None
This tells that the function is indeed meant to return a value for later use, and in this case it returns None. This value None can then be used elsewhere. return None is never used if there are no other possible return values from the function.
In the following example, we return person's mother if the person given is a human. If it's not a human, we return None since the person doesn't have a mother (let's suppose it's not an animal or something).
def get_mother(person):
if is_human(person):
return person.mother
else:
return None
Using return
This is used for the same reason as break in loops. The return value doesn't matter and you only want to exit the whole function. It's extremely useful in some places, even though you don't need it that often.
We've got 15 prisoners and we know one of them has a knife. We loop through each prisoner one by one to check if they have a knife. If we hit the person with a knife, we can just exit the function because we know there's only one knife and no reason the check rest of the prisoners. If we don't find the prisoner with a knife, we raise an alert. This could be done in many different ways and using return is probably not even the best way, but it's just an example to show how to use return for exiting a function.
def find_prisoner_with_knife(prisoners):
for prisoner in prisoners:
if "knife" in prisoner.items:
prisoner.move_to_inquisition()
return # no need to check rest of the prisoners nor raise an alert
raise_alert()
Note: You should never do var = find_prisoner_with_knife(), since the return value is not meant to be caught.
Using no return at all
This will also return None, but that value is not meant to be used or caught. It simply means that the function ended successfully. It's basically the same as return in void functions in languages such as C++ or Java.
In the following example, we set person's mother's name and then the function exits after completing successfully.
def set_mother(person, mother):
if is_human(person):
person.mother = mother
Note: You should never do var = set_mother(my_person, my_mother), since the return value is not meant to be caught.
Yes, they are all the same.
We can review the interpreted machine code to confirm that that they're all doing the exact same thing.
import dis
def f1():
print "Hello World"
return None
def f2():
print "Hello World"
return
def f3():
print "Hello World"
dis.dis(f1)
4 0 LOAD_CONST 1 ('Hello World')
3 PRINT_ITEM
4 PRINT_NEWLINE
5 5 LOAD_CONST 0 (None)
8 RETURN_VALUE
dis.dis(f2)
9 0 LOAD_CONST 1 ('Hello World')
3 PRINT_ITEM
4 PRINT_NEWLINE
10 5 LOAD_CONST 0 (None)
8 RETURN_VALUE
dis.dis(f3)
14 0 LOAD_CONST 1 ('Hello World')
3 PRINT_ITEM
4 PRINT_NEWLINE
5 LOAD_CONST 0 (None)
8 RETURN_VALUE
I have a function that fills out a dictionary with the values of specific variables as I iterate through another function to track what the actual output for each is. One of the things I'm using this for is to catch errors, so if a variable doesn't get assigned I get a value error and handle that, adding <no value> to the dictionary instead of the actual value. I don't want to use None or a blank string since those could actually be the value of one of these variables. <no value> seems fine for what I need to do, but I'm wondering if there's some agreed upon "null" value that I could use here instead
Why does the my code return none!? I’m a noob so I don’t know whether the question is overtly stupid or what😃
How to return Null in python
object oriented - Is it better to return NULL or empty values from functions/methods where the return value is not present? - Software Engineering Stack Exchange
Why does one version of my function return None, while the other works fine?
Videos
There is this one problem in leetcode which tells you to return null when there is an empty string, but then I don't know how to return Null, there aren't any resources that tell you how to return Null, but there are resources that tell you how to return nothing, but still that is not what I'm after.
StackOverflow has a good discussion about this exact topic in this Q&A. In the top rated question, kronoz notes:
Returning null is usually the best idea if you intend to indicate that no data is available.
An empty object implies data has been returned, whereas returning null clearly indicates that nothing has been returned.
Additionally, returning a null will result in a null exception if you attempt to access members in the object, which can be useful for highlighting buggy code - attempting to access a member of nothing makes no sense. Accessing members of an empty object will not fail meaning bugs can go undiscovered.
Personally, I like to return empty strings for functions that return strings to minimize the amount of error handling that needs to be put in place. However, you'll need to make sure that the group that your working with will follow the same convention - otherwise the benefits of this decision won't be achieved.
However, as the poster in the SO answer noted, nulls should probably be returned if an object is expected so that there is no doubt about whether data is being returned.
In the end, there's no single best way of doing things. Building a team consensus will ultimately drive your team's best practices.
In all the code I write, I avoid returning null from a function. I read that in Clean Code.
The problem with using null is that the person using the interface doesn't know if null is a possible outcome, and whether they have to check for it, because there's no not null reference type.
In F# you can return an option type, which can be some(Person) or none, so it's obvious to the caller that they have to check.
The analogous C# (anti-)pattern is the Try... method:
public bool TryFindPerson(int personId, out Person result);
Now I know people have said they hate the Try... pattern because having an output parameter breaks the ideas of a pure function, but it's really no different than:
class FindResult<T>
{
public FindResult(bool found, T result)
{
this.Found = found;
this.Result = result;
}
public bool Found { get; private set; }
// Only valid if Found is true
public T Result { get; private set;
}
public FindResult<Person> FindPerson(int personId);
...and to be honest you can assume that every .NET programmer knows about the Try... pattern because it's used internally by the .NET framework. That means they don't have to read the documentation to understand what it does, which is more important to me than sticking to some purist's view of functions (understanding that result is an out parameter, not a ref parameter).
So I'd go with TryFindPerson because you seem to indicate it's perfectly normal to be unable to find it.
If, on the other hand, there's no logical reason that the caller would ever provide a personId that didn't exist, I would probably do this:
public Person GetPerson(int personId);
...and then I'd throw an exception if it was invalid. The Get... prefix implies that the caller knows it should succeed.
Python and I are NOT getting along today.
I have the script below, which takes arguments like username and fromdate and passes it into the appropriate place in the URL (the link variable). Now, if the user just inputs username and doesn't input fromdate, Python will return None instead of nothing. Why do you do this, Python!? I never asked you to!
How can I avoid this monstrosity?
Command
main.py -u jacksfilms
Script
import requests, re, argparse
parser = argparse.ArgumentParser()
parser.add_argument('-u','--username', required=False)
parser.add_argument('-from','--fromdate', required=False)
parser.add_argument('-to','--todate', required=False)
args = vars(parser.parse_args())
username = args['username']
fromdate = args['fromdate']
todate = args['todate']
link = "https://web.archive.org/cdx/search/cdx?url=twitter.com/{}/status&matchType=prefix&from={}&to={}".format(username,fromdate,todate)
data = []
c = requests.get(link).text
urls = re.findall(r'https?://[^\s<>"]+[|www\.^\s<>"]+', c)
for i, url in enumerate (urls):
data.append(f"{i}: {url}\n")
print(data)